In describing the background of the invention it is the intention of the inventors to prime the reader for a greater understanding of the invention. In doing so certain documents, patents, etc. may be described. These documents speak for themselves, and although any discussion of their contents herein is believed to be accurate, any inadvertent mischaracterization does not constitute an admission.
Inkjet printing technology allows a base paper to be printed using droplets of ink. These are ejected or propelled from different mechanical means to a paper medium, on which they form points which create the corresponding text or image.
The intrinsic qualities of this technology account for its rapid development: possibility of high-speed, contact-free and impact-free printing, with high definition, thus giving access to high-quality colour images. Independently, progress made in the fields of electronics and computing is contributing to the constant improvement of computers and digital cameras; this development of the hardware, combined with the low purchase price of inkjet printers, is leading increasing numbers of users to take up this printing technology. This technology is currently used both by professionals and by the general public to print objects as diverse as letters, reports, brochures, magazines, postcards, digital photographs, labels, posters, etc.
There are currently considered to be 2 families of media for inkjet: “ordinary” papers and “special” papers. The former are used for more inexpensive low- to medium-quality printing. The latter are recommended when a high rendering of the image or of the text to be printed is expected, and they are obtained at higher costs. The major difference between these 2 categories lies in the application of a water-based coating to the surface of the medium:                of low coat weight and with inexpensive organic and mineral compounds in the case of ordinary papers;        of higher coat weight and with more sophisticated and expensive organic or mineral compounds in the case of special papers.        
This coating is called a “paper coating”: it is an aqueous formulation, conventionally containing water, at least one mineral filler, one or more binders, and also various additives. In paper coatings intended for inkjet-type printing, either mineral or organic salts, or cationic species, can be introduced, the primary function of which is to fix the ink at the moment when the latter reaches the surface of the paper in the form of droplets. Documents WO 2009/110910, WO 2010/068193 and WO 2011/008218 give two examples of such formulations containing salts, while document WO 2007/112013 concerns paper coatings containing cationic species; all these documents relate to inkjet technology.
The mineral filler introduced into the paper coating is conveyed in the form of an aqueous suspension. Conventionally, this filler is a calcium carbonate, suspended in water by means of a dispersing agent.
The most widespread dispersing agents are anionic polymers of low molecular weight (less than 10,000 g/mol) which are homopolymers of acrylic acid. With this regard, patent applications FR 2 488 814, FR 2 603 042, EP 0 100 947, EP 0 100 948, EP 0 129 329, EP 0 542 643 and EP 0 542 644 may be cited. But weakly anionic dispersing agents, which are copolymers of low molecular weights of acrylic acid, can also be used with a monomer of the methoxy or hydroxy oxyalkylated type; examples of such will be found in document WO 01/096007.
This being so, when the formulation of the paper coating is produced by blending between the different constituents listed above, the introduction of the salt or cationic compound may cause a problem when a rheology agent, the function of which is to regulate the viscosity, and to improve the water retention of the said coating, is introduced in addition. These two parameters improve control of the coating method, spreading quality on the base paper and maintenance of the active compounds of the paper coating on the surface of the paper.
Conventional rheology agents are generally anionic polymers, concerning which it has been observed that introducing them into coatings for inkjets containing salts or a cationic compound causes a violent flocculation reaction, shown by a high rise in viscosity, without any noteworthy improvement of water retention.